This invention relates generally to an electrophographic printing machine, and more particularly concerns a platen for use therein:
Generally, electrophotographic printing machines are provided with a transparent platen upon which the original document is supported. An optical image of the original document is created and recorded as an electrostatic latent image upon a photoconductive surface. The machine is usually provided with a cover to prevent extraneous light from entering the optical system during the imaging process. Typically, the cover has a rigid outer shell with a suitable light weight rubber material adhering thereto and arranged to engage the original document disposed on the platen. The rubber material is flexible and colored white to reflect light rays therefrom. U.S. Pat. No. 3,642,371 issued to Jones et al. in 1972, disclosed a platen cover having a white reflective surface.
The basic operation of an electrophotographic printing machine is well known and this concept has been employed in a wide variety of products. Various special purpose types of electrophotographic printing machines have been developed and are in wide commercial use. For example, electrophotographic printing machines which employ a field lens have been developed for reproducing microfilm. Machines of this type are described in U.S. Pat. No. 3,424,525 issued to Towers et al. in 1969; U.S. Pat. No. 3,542,468 issued to Blow, Jr. in 1970; and U.S. Pat. No. 3,547,533 issued to Stokes et al. in 1970. In general, a microfilm reproducing machine produces an enlarged copy of a microfilm original. However, it has been found that it is frequently difficult to produce copies having pictorial quality. Only recently have electrophotographic printing machines been developed which have the capability of reproducing slides as pictorial quality copies. A machine of this type is described in co-pending application Ser. No. 540,617 filed in 1975, now U.S. Pat. No. 4,027,962, issued to Mailloux in 1977. As described therein, a light image of a transparency is projected onto a mirror. The mirror reflects the light image through a screen and field lens onto the charged portion of the photoconductive surface. The screen and field lens are disposed on the platen of the electrophotographic printing machine. Generally, the platen is a planar transparent glass member. The light image transmitted through the platen irradiates the charged photoconductive surface. In this manner, an electrostatic latent image is recorded on the photoconductive surface and developed with toner particles. The toner powder images are transferred to a sheet of support material and then permanently affixed thereto forming a copy of the slide being reproduced.
In reproducing slides, both the screen and field lens are disposed on the transparent platen. Contrawise, in reproducing opaque original documents, only the original document is disposed on the transparent platen with both the screen and field lens being removed therefrom. This introduces additional manipulations and frequently results in the loss of either the screen or field lens. It would appear to be highly desirable to integrate the field lens with the structure of the electrophotographic printing machine to eliminate the requirement of moving the field lens into and out of the optical light path depending upon the type of original document being reproduced.
Accordingly, it is a primary object of the present invention to improve the platen structure of an electrophotographic printing machine to reproduce transparencies and opaque documents thereon.